OK! I just did what I should have done before I opened this topic. I searched Profile's product site.
It turns out that the correct name is "Aquatic Plant Soil". I found it in the "lawn & garden" section.
Apparently, it is inert, neutral, and will not break down or leach. It "stores and exchanges nutrients directly to aquatic plants".
Though it's obviously intended for outdoor ponds it sounds ideal for my application.